ACQUIRED MELANOCYTIC NEVI
Acquired melanocytic nevocellular nevi are small (<1 cm), benign, well-circumscribed, pigmented lesions comprised of groups of melanocytes or melanocytic nevus cells.
They can be classified into three groups:
Junctional nevi (cells grouped at the dermal–epidermal junction, above the basement membrane).
Dermal nevi (cells grouped in the dermis).
Compound nevi (combination of histologic features of junctional and dermal).
Clinical overlap exists among all three types.
Only some 30% of melanomas arise from pre-existing nevi; thus prophylactically removing all the nevi on a person is neither warranted nor protective.
SYNONYMS Pigmented nevi, nevocellular nevus, moles.
AGE Nevi appear after 6 to 12 months of age, peak during the third decade, and then slowly disappear.
INCIDENCE Common. By age 25, most Caucasians will have 20 to 40 moles.
RACE Caucasians have more total body nevi than darker skin types. Asians and blacks have more nevi on atypical locations (palms, soles, nail beds, and conjunctivae) than whites.
GENETICS Increased number of nevi tend to cluster in families. Increased clinically atypical nevi may be more prevalent in families with melanoma.
DURATION OF LESIONS Commonly called moles, lesions appear after the age of 6 to 12 months and reach a maximum number between ages 20 and 29. By age 60, many moles fade and/or disappear.
SKIN SYMPTOMS Nevocellular nevi are asymptomatic. If a mole is symptomatic, it should be evaluated and/or removed.
Melanocytic nevi need to be differentiated from seborrheic keratoses, dermatofibromas, neurofibromas, fibroepithelial polyps, basal cell carcinomas, and melanomas.
Indications for removal of acquired melanocytic nevi are the following:
Asymmetry in shape. One-half is different from the other.
Border. Irregular borders are present.
Color. Color is or becomes variegated. Shades of gray, black, white are worrisome.
Diameter. Greater than 6 mm (may be congenital mole, but should be evaluated).
Evolution: If the lesion is growing rapidly, distinct from other nevi or a child's overall growth pattern.
Symptoms. Lesion begins to persistently itch, hurt, or bleed.
Site. If the lesion is repeatedly traumatized in any given location (e.g., waistline, neck) or if the lesion is in a high-risk/difficult-to-monitor site such as the mucous membranes or anogenital area, it may warrant removal.
These criteria are based on anatomic sites at risk for change of acquired nevi to malignant melanoma or on changes in individual lesions (color, border) that indicate the development of a focus of cells with dysplasia, the precursor of malignant melanoma. Dysplastic nevi are usually >6 mm, and darker, with a variegation of color (tan, brown), and irregular borders. Approximately one-third of melanomas are associated with precursor nevi, and an increased number of nevi increases the melanoma risk.